In preparing a thin film magnetic head, upper and lower magnetic layers of sendust or amorphous materials are made on a substrate formed of a wear-resistant material, such as ferrite or sapphire. A coil conductor layer of conductive metals and an insulating layer are formed between these magnetic layers and subjected to repeated etching for patterning to a predetermined contour (pattern). Finally, a protective layer is formed for protecting the magnetic layers against wear caused by the running of the magnetic recording medium. Alumina or SiO.sub.2 have been used as the material for the protective layer.
In the above construction, it is well-known that, when the protective layer is sufficiently harder than the magnetic layers, the magnetic layers undergo premature wear due to sliding contact with the running recording medium. This results in the partially advancing wear (partial abrasion) of the magnetic layers, which leads to a spacing loss.
On the other hand, if the protective layer is too soft, the magnetic head in its entirety tends to undergo premature wear, resulting in a shortened service life and decreased durability. It is therefore preferred for the hardness of the protective layer to be approximately equal to or slightly lower than that of the magnetic layers.
Also, in view of the sliding properties and the resistance to the recording medium and the partially advancing wear, the protective layer needs to be 20 to 40 .mu.m or more in thickness. However, with such a film thickness, the protective layer usually undergoes peeling or cracking due to the cumulative internal stress. For this reason, the internal stress needs to be reduced to a value as low as possible. One effective method of obviating this drawback would be match to the coefficients of thermal expansion of the materials making up the magnetic head. However, it is generally difficult with conventional techniques to effect matching the coefficients of thermal expansion of the magnetic metal materials and the protective layer.
In the Japanese Patent KOKAI Publication No. 62-16218 (1987), a mixture of MgO and SiO.sub.2 is disclosed as a protective layer for effectively suppressing such partially advancing wear. It is also described therein that a moderate hardness Hv of 450 to 850 kgf/mm.sup.2 and a desired coefficient of thermal expansion may be realized by selecting the compositional ratio of MgO to SiO.sub.2 so that the SiO.sub.2 has a concentration of 10 to 70% in the MgO/SiO.sub.2 mixture.